Set Default Arguments with Bash Shell Parameter Expansions

Cameron Nokes
InstructorCameron Nokes

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In this lesson, we'll see how shell parameter expansions can be used to simply expand a variable's valuable and also provide a default value to a variable, if not set. Note that there are many more possibilities with shell parameter expansions, so check bash's documentation to view them all.

Eliezer Steinbock
Eliezer Steinbock
~ 2 years ago

Why does dir=$1 turn into: dir=${1:-$PWD}. I'm a little confused by the brace ordering. I would have expected something like ${$1 instead of ${1

Cameron Nokes
Cameron Nokesinstructor
~ 2 years ago

Why does dir=$1 turn into: dir=${1:-$PWD}. I'm a little confused by the brace ordering. I would have expected something like ${$1 instead of ${1

I agree the syntax is strange and I don't think I can give you a satisfactory answer without knowing how the bash interpreter works 🤷‍♂️. Remember that the syntax $var is basically equivalent to ${var}, so syntactically, a variable in bash isn't always preceded by a $. The "why" might be covered here more: http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_04.html. Sorry, I couldn't be of more help and that's definitely a good question!